1. Field of the Invention
This invention relates to a package for a semiconductor device, and more particularly to a heat transfer structure in the package for the semiconductor device.
2. Description of the Related Art
Semiconductor integrated circuit chips are required to have a high integration level so as to improve their performance. Further, multi-chip packaging technologies are utilized to permit semiconductor integrated circuit chips to have a high density in order to shorten a length of an interconnection between the semiconductor integrated circuit chips. Thus, a package, particularly a multi-chip module, includes a plurality of semiconductor integrated circuit chips. In accordance with the above requirement, the package of the semiconductor integrated circuit chips has a disadvantage in a heat accumulation in the package. The accumulation of the heat in the package makes for poor performances and reliability of the integrated circuits. Particularly, as high speed performance and a high integration of the semiconductor integrated circuit, chips are required, the above problem in the heat generation is more serious. Whether the high integrated circuit chips are able to exhibit excellent performances such as a high speed performance and to have a high reliability depends upon the heat transferring structure. It is, thus, very important for the high density packaging technology how to remove the heat generated by the semiconductor integrated circuit chips to the exterior of the package.
FIG. 1 is a cross sectional view showing the prior art. In the prior art, a plurality of semiconductor integrated circuit chips 21 are placed on a mounting substrate 23 through bumps 22 by use of a flip chip bonding method and the like to be placed in a package 24. The mounting substrate 23 is electrically connected to leads 25 provided to the package 24 through metal wires 26. A cap 27 is attached to a surface of each of the semiconductor integrated chips 21 to seal the package 24. A heat sink is provided on the cap 27.
According to the prior art, the heat generated from the semiconductor integrated circuit chips 21 can be removed through the cap 27 and the heat sink 28 to the exterior of the package 24.
The prior art is, however, engaged with a disadvantage as follows. A face of each of the semiconductor integrated circuit chips 21 is in contact with one surface of the cap 27 whose opposite surface is in contact with the heat sink 28. The semiconductor integrated circuit chips 21, the cap 27 and the heat sink 28 have different coefficients of thermal expansion respectively, for which reason a stress occurs between those, thereby resulting in an appearance of a disconnection between the semiconductor integrated circuit chips 21 and the mounting substrate 23, and also in an appearance of a crack in the semiconductor integrated circuit chips 21.
To settle the above issues, another heat transfer structure is disclosed in the Japanese laid open patent application No. 63-287038. FIG. 2 is a cross sectional view showing this other prior art.
In this other prior art, a plurality of semiconductor integrated circuit chips 31 are placed on a mounting substrate 33 through solder 32 to be sealed by a cap 37 made of a material having a high thermal conductivity, such as copper and the like. A high thermal conductive plate 34 is attached to a face of the semiconductor integrated circuit chip 31. The semiconductor integrated circuit chip 31 and the cap 37 are connected through a heat conductor 35 which comprises flexible metal fibers. The cap 37 is attached to the mounting substrate 33 by metal bellows 36.
In such structure, the semiconductor integrated circuit chip 31 and the cap 37 are connected through the high thermal conductive plate 34 and the heat conductors 35 so that the heat generated from the semiconductor integrated circuit chip 31 can easily be transferred to the cap 37 to radiate it to the exterior. In addition, the heat conductor 35 comprising the flexible metal fibers. Thus, a thermal stress occurring between the semiconductor integrated circuit chip 31 and the cap 37 due to their different coefficient of thermal expansion can be absorbed and mitigated in the heat conductors 35. Namely, the heat conductors 35 can prevent the semiconductor integrated circuit chip 31 from cracking and also prevent the connection the semiconductor integrated circuit chip 31 with the mounting substrate 33 from breaking.
Further, the metal bellows 36 comprises metal thin plates. The metal thin plates are attached to each other for formation of the metal bellows 36. Therefore, the metal bellows 36 is able to absorb or mitigate a stress occurring between the cap 37 and the mounting substrate 33 corresponding to a relative displacement between them.
This other prior art is, however, engaged with a disadvantage in the following. The heat conductor 35 is required to comprise thin metal fibers spaced from each other and to leave a space for high flexibility. An effective area of the heat conductors 35 is small, thereby a thermal resistance of the heat conductors 35 is high to result in a low heat radiation efficiency.
If thick metal fibers are provided as the heat conductor 35 at a narrow space for obtaining a high heat radiation, a rigidity of the heat conductors 35 becomes high. This makes it difficult to obtain a high flexibility. It is, therefore, difficult to absorb or mitigate a thermal stress occurring in the semiconductor integrated circuit chips 31. Further, a fabricating process of the heat conductor 35 becomes complicated and its cost rises since it is required to have a predetermined space between the metal fibers and to have a predetermined length in the metal fibers.